The present invention relates to electrostatic chucks and a process for producing the same.
Heretofore, electrostatic chucks have been used to adsorb and hold semiconductor wafers in various steps represented by transferring, light exposing, film-forming such as CVD, washing, and fine processing such as etching and dicing for such semiconductor wafers. A number of projections or embossed portions are usually provided projecting from a setting face of an insulating layer of the electrostatic chuck, so that a semiconductor wafer is contacted with top surfaces (contact surfaces) of the projections. Further, DC voltage is applied to an internal electrode inside the insulating layer so that Johnson-Rahbeck force may be generated at a contact interface between contacting surfaces of the semi-conductor wafer and the projections, thereby adsorbing the semi-conductor wafer on the contacting surface. Therefore, the adsorbing force for the semiconductor wafer can be increased by enlarging the area of the contacting surfaces (top surfaces) of the projections.
However, the electrostatic chuck has a problem in that particles are formed at a side of a back side surface (a surface to be adsorbed) of the wafer. The particles are principally particles which are formed when silicon is ground after they are attached to a silicon wafer, particles of aluminum nitride which come off from the surface of the electrostatic chuck, particles of organic material which remain on the surface of the electrostatic chuck, etc.
The particles are generated only when the wafer is adsorbed onto the electrostatic chuck. However, particularly when a low-temperature wafer is adsorbed onto the electrostatic chuck held at a high temperature or when heat is inputted from plasma to the wafer in the state that the low-temperature wafer is adsorbed onto the electrostatic chuck at a low temperature, the wafer is heated and thermally expands while an adsorbing force is being generated for the wafer. Consequently, the back side surface of the wafer rubs the surface of the electrostatic chuck. In such a case, particles are likely to be formed.
JP-A 7-245336 discloses that when a silicon wafer contacts uneven portions at the adsorbing surface of the ceramic electrostatic chuck, the silicon wafer having a relatively low hardness is ground by these uneven portions to generate particles and that such uneven portions are ground by irradiating plasma upon the adsorbing surface of the electrostatic chuck, thereby rounding fine projections and reducing the formation of the particles.
JP-A 8-55900 discloses a method in which when a silicon wafer is to be adsorbed onto an electrostatic chuck, voltage to be applied to the electrostatic chuck is gradually increased to mitigate impact occurring when the silicon wafer contacts the electrostatic chuck and reduce the generation of particles resulting from uneven portions at an adsorbing surface of the electrostatic chuck.
In order to reduce the generation of the particles, the contact area between the back side surface of the wafer and the corresponding surface of the electrostatic chuck has been reduced. However, this method has a problem. That is, since a backside gas is ordinarily flown between the wafer and the electrostatic chuck, the wafer is floated with the pressure of the backside gas, if an adsorbing force is weak for the wafer. Therefore, the adsorbing force for the wafer must be sufficiently higher than the pressure of the backside gas.
The total adsorbing force is a product between the contacting area and an adsorbing force per unit area at the contacting portion. Therefore, if the contacting area between the back side surface of the wafer and the surface of the electrostatic chuck is reduced, the adsorbing force per unit area needs to be increased accordingly. For this purpose, the voltage to be applied needs to be increased, for example. However, if the adsorbing force per unit area is increased, shearing stress applied to the wafer increases when the wafer is thermally expanded after being adsorbed. As a result, particles are generated on the wafer.
If aluminum nitride particles are peeled from the wafer, such peeled particles become particles in concern, which not only interrupt wiring of the chip circuit and reduce the production yield of chips, but also cause changes in the wafer-adsorbing characteristics of the electrostatic chuck and make the wafer-treating process unstable.
It is an object of the present invention to provide an electro-static chuck provided with an dielectric layer made of aluminum nitride upon which a wafer is to be adsorbed, the electrostatic chuck being able to prevent the generation of particles resulting from thermal expansion of the wafer after being adsorbed onto the electrostatic chuck.
The invention also provides a method for producing such an electrostatic chuck.
The electrostatic chuck according to the present invention comprises a dielectric layer made of aluminum nitride, and a surface layer covering a surface of the dielectric layer, made of a material harder than the aluminum nitride constituting the dielectric layer and having a thickness of not less than 200 nm, said surface of the dielectric layer having a center-line average surface roughness of not more than 25 nm, and said electrostatic chuck being adapted to adsorb a wafer onto the dielectric layer through the surface layer.
The electrostatic chuck producing method according to the present invention comprises the steps of preparing the dielectric layer made of the aluminum nitride, adjusting a surface of the dielectric layer to a center-line average surface roughness of not less than 25 nm, and covering the surface of the dielectric layer with not less than 200 nm in thickness of a material harder than said aluminum nitride constituting the dielectric layer.